This invention relates to operation control systems for internal combustion engines having throttle valves.
If an operating mechanism for operating a throttle valve arranged in the intake system of an internal combustion engine is constructed to vary the valve opening of the throttle valve linearly in proportion to an amount of stepping on the accelerator pedal, even a slight change in the stepping amount of the accelerator pedal will cause an abrupt change in the quantity of intake air supplied to the engine when the engine is operating in a low load region, often spoiling the driveability of the engine. This is conspicuous particularly in a so-called multi-throttle engine in which a plurality of throttle valves are arranged in respective intake passages (branch passages) of the intake manifold which are connected to respective cylinders so as to vary the quantities of intake air supplied to the respective cylinders in quick response to changes in the throttle valve openings for improved driveability of the engine at sudden acceleration or at sudden deceleration. However, since the throttle valves are thus located closer to respective intake valves than in an ordinary type engine, the passage volume of a portion of the intake passage extending between each throttle valve and the associated intake valve is smaller than that in the ordinary type engine. As a consequence, even a slight change in the valve opening of the throttle valve will cause a large change in the intake air quantity supplied to the engine, which makes the intake air quantity change the more conspicuous in response to a change in the stepping amount of the accelerator pedal.
FIG. 1 shows changes in the throttle valve opening .theta.TH and the intake air quantity Qa which are plotted with respect to the stepping amount .theta.ACC of the accelerator pedal. It will be learned from the graph that if the throttle valve opening .theta.TH is varied along a straight solid line a, i.e. in direct proportion to the accelerator pedal stepping amount .theta.ACC, the resulting intake air quantity Qa varies along a solid line A, that is, the rate of change in the intake air quantity is larger when the accelerator pedal is in a smaller stepping-amount region L, i.e. when the engine is in a low load region.
To eliminate the aforementioned disadvantage, it is therefore desirable that in the low load region L of the engine the intake air quantity Qa should vary linearly (along a straight dotted line B in FIG. 1) in direct proportion to the accelerator pedal stepping amount .theta.ACC. To this end, the throttle valve operating mechanism should be designed so as to reduce the rate of change in the throttle valve opening .theta.TH with respect to the change of the accelerator pedal stepping amount .theta.ACC in the low load region L of the engine. However, conventionally a throttle valve opening sensor as a component of an engine operation control system such as an electronic fuel supply control system is mounted on the valve stem of the throttle valve to sense the throttle valve opening .theta.TH. If the sensor is used together with the throttle valve operating mechanism designed to reduce the rate of change in the throttle valve opening in the low load region L, there will be a time delay in detecting sudden acceleration of the engine from an idling region due to the reduced rate of change in the throttle valve opening in the low load region L, resulting in degraded driveability of the engine.